The receptor-type protein tyrosine phosphatase, PTPRO, is known to induce cell contact inhibition, cell cycle arrest, terminal cell differentiation and apoptosis in human cancer cell lines, particularly leukemia cells. Our study has shown that PTPRO is suppressed by hypermethylation in human primary tumors (hepatocellular, lung and CLL) as well as leukemia and lung cancer lines, and that ectopic expression of the full length form PTPRO-FL in non-expressing cells inhibited anchorage-independent growth, delayed re-entry of the cells into cell cycle, increased susceptibility to apoptosis and inhibited tumor growth in nude mice. Further, PTPRO is localized to chromosome 12p12.3 that is characterized by loss of heterozygosity in a variety of human cancer, a characteristic of many tumor suppressor genes. In addition, patients exhibiting PTPRO promoter methylation had higher expression of at least three anti-apoptotic proteins (Bcl2, Mcl-1, XIAP) independent of other commonly known prognostic factors including interphase cytogenetics, VH and p53 mutational status. The hypothesis of this project is that PTPRO has the potential for functioning as a growth/tumor suppressor that could be utilized as a novel molecular target in cancer, particularly CLL, therapy. The specific aims are to ( 1) Investigate whether PTPROt (predominant form in cells of lymphoid origin) expression is down-regulated in chronic lymphocytic leukemia (CLL), whether PTPRO promoter methylation identifies a subset of high risk group of CLL patients, and whether the suppression of PTPROt correlates inversely with the methylation status/density of the CpG island located in the promoter (2) confirm the growth/tumor suppressor property or anti-transformation potential and pro-apoptotic property of PTPROt isoforms, (3) identify the substrate(s) of the PTPROt isoforms and (4) elucidate the molecular mechanism by which methylation suppressed expression of PTPRO by (a) exploring the chromatin structure of PTPRO promoter (b) investigating whether the novel post-translational modifications of core histones (identified in Project 4) are associated with the PTPRO promoter in normal B lymphocytes and whether this association is altered in CLL(c) determining the involvement of DNA methyltransferases, methyl CpG binding proteins, and chromatin remodelers (studied in Project 5) in regulating PTPRO expression in CLL cells. Project 1 will interact with this project in studies on re-activation of the suppressed genes by agents that inhibit DNA methyltransferases and histone deacetylases. It is hoped that this study will provide a novel molecular target in CLL therapy and molecular marker for CLL, and could reveal the potential for drug resistance in a subset of CLL patients with PTPRO methylation independent of other commonly known prognostic markers.